In a radiography study, an object to be examined, typically a patient is placed between a source of x-rays and a fluorescent screen or photographic film. The x-rays passing through the object produce light photons when they strike the fluorescent screen or silver grains in the developed film.
For a given cross section of fluorescent screen it is possible to calculate a brightness factor which can be expressed in units of light photons per square millimeter per second. This brightness factor could, of course, also be expressed in other equivalent units such as photons per square meter per second. An intensifier is any device which when used in an x-ray system increases the number of light photons produced per square millimeter per second.
It is known that such an intensifier can improve x-ray image brightness thereby enhancing viewing comfort. The reasons for this improvement in x-ray viewing are well documented in the literature, c.f. Encyclopedia of X-Rays and Gamma Rays ed. by George C. Clark, Reinhold Publishing Corporation copyright 1963 at page 503.
The principal ingredient in most commonly used intensifier units is an electron tube known by such diverse names as image phototube, image converter, image tube, or image intensifier tube. They all refer to an evacuated envelope which houses apparatus for increasing the brightness of the image generated in response to the x-radiation.
In a typical intensifier, the x-radiation passes through a glass or metal envelope and falls upon an input screen which includes a fluorescent layer. Visible light energy produced by the fluorescent layer passes to a photoemissive layer in the input screen which cause photoelectron emission. The photoelectron density corresponds to the brightness distribution over the fluorescent layer which in turn corresponds to the x-ray intensity over the examined patient area.
At an opposed end of the intensifier tube is positioned an output screen. Application of suitable accelerating potentials to the photoelectrons from the photoemissive layer causes those electrons to strike the output screen. This output screen also includes a fluorescent material layer so again a visible image is generated. In a typical application a 30 kv power supply is used to provide the requisite acceleration.
A number of control electrodes are typically positioned along the acceleration path to cause the image on the output screen to be reduced in size from the image at the input screen. Both input and output screens include conductive layers which when coupled to the power supply separate those screens by a large potential difference. Due to the electrical interaction between the electrons and the electrode/screen combination the image at the output screen is much brighter than an image attained by direct fluroscopy. Brightness enhancement of several orders of magnitude is possible with such a unit. The reduced image may be subsequently enlarged without brightness diminution.
Causing electrons to accelerate between an input screen's electron emissive layer and an output or viewing screen enhances image brightness but at the expense of producing an image anomaly or abberation called S-distortion. S-distortion is caused by a magnetic field exerting a force on the moving electrons inside the intensifier tube. This force causes a straight line in the input image to be imaged as an S-shaped curve at the output image and thus the name S-distortion of S'ing. This same magnetic force also causes the output image to be rotated with respect to the input image. The amount of magnetic distortion (S'ing and image rotation) depends on the magnetic field strength inside the intensifier tube, the orientation of the tube with respect to the magnetic field direction, and the size of the input image. Magnetic distortions are more detrimental to image quality for larger diameter intensifying tubes. With increasing use of larger input screen intensifier tubes the magnetic distortion problem becomes even more significant.
The natural magnetic field associated with the earth is a vector field with the flux lines running from the southern to the northern hemisphere. These lines intercept the earth's surface at an angle called the dip angle. One publication, the Handbook of Chemistry and Physics, lists the horizontal component of the earth's magnetic field vector and the dip angle for various locations around the world.
The force exerted on a charged particle in an electric field (E) and magnetic field (B) is given by the Lorentz equation: EQU F=F electric+F magnetic=qE+qv.times.B (1)
where q and v are the charge and the velocity of the charged particle.
In an intensifier tube the charged particles are electrons which are acted upon an E field set-up by the charged electrode structure and an undesirable B field from the natural magnetic field about the earth. Since the magnetic force is EQU F magnetic=q (v.times.B)
this force is proportional to the velocity of the electron and the direction of the force is perpendicular to both the velocity and the B field. This is the force that causes the magnetic S'ing distortions in the image.
Suggestions have been made to reduce S'ing and image rotation caused by the earth's magnetic field. One technique use in prior art tubes is the provision of a mu-metal shield around the tube designed to shield the interior from magnetic fields. The shield, in theory, draws the earth's magnetic flux into itself and thus deflects the magnetic flux lines away from the interior of the intensifier tube. The mu-metal shield must be open, however, at the input and output ends of the intensifier tube and therefore some part of the earth's magnetic field enters the tube and results in S-distortion.
A second technique used to reduce S'ing and image rotation also modifies the magnetic field within the tube. One system embodying the second technique is disclosed in U.S. Pat. No. 3,809,889. This patent shows an intensifier tube with a current carrying coil positioned about the exterior of the tube near the input screen. When a current passes through the coil a magnetic field is created in the vicinity of the input screen which tends to cancel the earth's magnetic field at that location.
Tests conducted using the configuration shown in U.S. Pat. No. 3,809,889, however, show that the magnetic field inside the tube is not effectively cancelled along the entire length of the tube. While the field near the input screen is reduced it should be recalled that the force on a moving electron is proportional to its velocity. Thus, as the electron is accelerated away from the U.S. Pat. No. 3,809,889 coil the magnetic side force is again experienced. In fact, the magnitude of this side force increases after the electron has picked up speed along its path toward the viewing screen. It is apparent therefore that the U.S. Pat. No. 3,809,889 arrangement with its S-coil near the input screen inadequately addresses the magnetic distortion problem.
The apparatus disclosed in U.S. Pat. No. 3,809,889 also ineffectively changes the current through its coil as the intensifier tube is re-oriented in the earth's field. The U.S. Pat. No. 3,809,889 apparatus comprises a Hall effect probe which measures changes in magnetic field in proximity to the intensifier tube. The current in the coil is then modified in response to the output of the hall effect device. This technique is inadequate. It is the field inside the tube which causes the distortion and not the field in proximity to the outside of the tube. The intensifier tube housing may disrupt the magnetic field and greatly alter the field on the inside of the tube. It is apparent that the U.S. Pat. No. 3,809,889 apparatus is unsuited to modify the current through its coil as the intensifier tube is re-oriented since the Hall probe is not measuring the correct magnetic field.